In mining applications it is frequently necessary to provide positioning cylinder assemblies, e.g. for the advance of a tool, for control of the stroke of an implement, for properly positioning a prop or the like or for a variety of other purposes.
For the most part, it is important that the one or more positioning cylinder assemblies have a position measuring device or means capable of signaling to a remote location, for control or display, the displacement of the piston within the cylinder.
For that purpose, positioning cylinder assemblies have been developed in which a piston carrying a piston rod is displaceable in the cylinder by a fluid medium, a spindle is rotatable in the cylinder and has a helical formation engaged by a nut or the like connected to the piston rod and the spindle is connected, in turn, through a friction clutch with a rotary potentiometer.
The axial displacement of the piston rod relative to the cylinder causes the nut to be axially displaced relative to the spindle and thus rotatably drives the spindle with a number of revolutions which depends upon the axial displacement of the piston.
The conventional positioning cylinder of this type has the spindle and the input shaft of the rotary potentiometer coaxial and synchronously driven. The friction coupling between them comprises a sleeve-like component which is connected with the spindle and a connecting pin engageable in the sleeve-like component and forming an extension of the input shaft of the rotary potentiometer. The connecting pin has an elastically yieldable coupling ring which functions as the friction clutch. The friction clutch is provided to allow a zero setting of the potentiometer to be matched with a zero or starting position of the piston or cylinder. This can be achieved by hand. The earlier positioning cylinder as described has been found to be satisfactory by and large. However, it has been found that the elastomeric coupling ring is sensitive to aging and, upon uncontrolled aging, the frictional connection can be altered. As a consequence, the zero positions can change and the accuracy of the signalled measurement can decrease.
Furthermore, in subterranean mining applications, positioning cylinders of different strokes are necessary. If a rotary potentiometer with a limited rotary stroke is provided with a friction clutch it is frequently necessary to adapt it to the positioning cylinder by replacing the spindle by utilizing a spindle and nut of a different pitch. The replacement of a spindle and nut in the cylinder assembly is an expensive and time consuming proposition.
In general, the cost can be reduced by utilizing as the spindle, a square-cross section bar which is provided with its helical formations by twisting the bar so that the edges thereof define the screw threads of helical pattern. For cylinders of different strokes, the different pitches are established by different degrees of twist. Such spindles, however, are not exact and thus the cost saving through the use of such spindles must be at the expense of precision. Indeed, in modern measurement and controlled systems, the inaccuracies introduced by such spindles are no longer acceptable.